This invention relates to an automatic polarization controlling device for use in an optical communication system, an optical fiber gyroscope, a like optical fiber sensor, and other polarization-sensitive optical apparatus.
In various fields of application of a coherent polarized beam, it is often necessary to carry out polarization control or stabilization so as to convert an variably polarized beam to a desirably polarized beam. Particularly in an optical heterodyne communication system, the polarization control is important because coincidence is indispensable between polarization states of a signal beam and a local oscillation beam which are mixed together into an optical beat signal. More particularly, a stabilization scheme is necessary in this instance in either aligning the polarization state of the signal beam with that of the local beam or in aligning the polarization state of the local beam with that of the signal beam. It is to be noted in this connection that the polarization state of the signal beam inevitably varies during propagation through a monomode or single-mode optical fiber due to temperature changes and other external disturbances. As a result, the optical beat signal has a fluctuating intensity. This gives rise to various problems such that the communication system has only a poor reliability and that detection of the signal beam becomes impossible in a worst case. For this reason, it is necessary to control a variably polarized beam to a desired polarized beam.
An automatic polarization controlling device is revealed in a letter contributed by R. Ulrich to Applied Physics Letters, Volume 35, No. 11 (Dec. 1, 1979), pages 840 to 842, under the title of "Polarization Stabilization on Single-mode Fiber." An electromagnet is used as a polarization controlling element in converting an element input polarized beam to an element output polarized beam in response to a driving voltage supplied to the polarization controlling element. Two electromagnets are used is series in squeezing a monomode optical fiber which is supplied with a device input polarized beam of an input polarization state and which produces a device output polarized beam of an output polarization state. The input polarization state is optionally variable. A portion of the device output polarized beam is directed by a beam splitter to a polarimeter which is used as a polarization monitor to monitor the output polarization state. The polarimeter controls a driver which produces the driving voltage for each electromagnet as an electric error signal. In other words, a feedback loop is used in controlling the output polarization state to a predetermined polarization state.
Each of the polarization controlling element and the driver is operable only in a certain voltage range. That is to say, the driving voltage must not exceed an upper and a lower range limit which usually have a common absolute value. When the input polarization state monotonically varies in one sense of rotation, the driving voltage would have to exceed the upper or the lower range limit. The polarization control becomes impossible. The driver is therefore reset according to Ulrich whenever the driving voltage of the driver under consideration approaches the upper or the lower limit. Immediately after the reset, the driver is restarted from a midrange. It is pointed out in the Ulrich letter that the reset is frequently necessary in a certain case. The frequent reset is reduced by a third electromagnetic fiber squeezer. Ulrich furthermore suggests use of a piezoelectric fiber squeezer as each polarization controlling element. At any rate, instantaneous interruption of the polarization control is unavoidable due to the reset before the driver again stably drives the polarization controlling element. The device of Ulrich is not stably operable continuously during a long term but is continuously operable only between two successive instantaneous interruptions.
Another automatic polarization controlling device is disclosed in a letter which was contributed by R. C. Alferness et al to Applied Physics Letters, Volume 38, No. 9 (May 1, 1981), pages 655 to 657, and entitled "Waveguide Electro-optic Polarization Transformer." Operation is analyzed in an article contributed by Rod C. Alferness alone to IEEE Journal of Quantum Electronics, Volume QE-17, No. 6 (June 1981), pages 965 to 969, under the title of "Electrooptic Guided-Wave Device for General Polarization Transformations." The device preferably comprises an x-cut lithium niobate or tantalate substrate which has a titanium-diffused waveguide and on which a first phase shifter, a mode converter, and a second phase shifter are integrated in cascade along the waveguide for use collectively as a polarization controlling element of a waveguide type in performing general polarization transformations. The waveguide supports a single TE or TM mode. Each phase shifter is voltage controlled to adjust a phase difference between TE and TM components of a polarized beam. The mode converter is for adjustably carrying out conversion between TE and TM modes under voltage control. The second phase shifter is unnecessary when a linear polarization state is desired as the output polarization state. At any rate, the instantaneous interruption is inevitable.
Still another automatic polarization controlling device is reported by W. A. Stallard et al as a report in the Third European Conference on Integrated Optics, held May 6-8, 1985, after the Convention date of the instant patent application. The report is recorded in Proceedings of the Conference edited by H. P. Nolting et al and published by Springer-Verlag, pages 164 et seq., particularly, pages 167 and 168. The device is a z-cut lithium niobate integrated optic device and is for use as an optical heterodyne detector. A polarization-dependent directional coupler is preferred by Stallard et al to a polarization-independent optical coupler for use in mixing the local oscillation beam with the signal beam after the polarization control. The device of Stallard et al is again susceptible to the instantaneous interruption.
In the manner exemplified above, the input polarization state may optionally vary. In this instance, the polarization control is to give a predetermined polarization state to the output polarization state. It may, however, be desirable as will be described later in the description to use an optional polarization state as a reference polarization state and to convert a predetermined polarization state to the optional polarization state.